The present invention relates to a motor-driven power steering system for an automobile, in particular, to a torque detector for such a system.
The prior art motor-driven power steering system for an automobile is so designed that the steering torque (i.e., the torque that develops on the steering wheel during the steering of a vehicle) is detected with a torque detector, with the motor being driven in response to an output signal from the detector in such a way that the steering shaft is subjected to power assist control.
FIG. 6 shows diagrammatically the composition of a motor-driven power steering system with which a torque detector is to be used. In this figure, 1 is a power steering motor, and 2 is an electromagnetic clutch mounted on the output shaft of the motor 1, with a first decelerating pinion 4 coaxially secured to the output shaft 3. A first gear wheel 5 meshing with the pinion 4 is coaxially secured to one end of an intermediate shaft 6, with a second decelerating pinion 7 being coaxially secured to the other end of the intermediate shaft 6. In FIG. 1, a pinion output shaft is shown by 9 and a second decelerating gear wheel 8 is coaxially secured to one end of the output shaft 9 and meshes with the second pinion 7. A plurality of first planetary gears 10 are coupled by pins 18 to a flange 9a formed at one end portion of the pinion output shaft. The front end portion of a torsion bar 11 is inserted into the end of the pinion output shaft, the two members being coupled by a pin 22. The rear end portion of the torsion bar 11 is inserted into a hollow portion 24 formed in the axial center of a sleeve shaft 12, the two members being coupled by a pin 23 at their rear end. A sun gear 13 is loosely fitted coaxially around the front end portion of the sleeve shaft 12 and meshes with the first planetary gears 10. The first planetary gears 10 in turn mesh with a first ring gear 15 the circumference of which is secured to a housing 21. A plurality of second planetary gears 14 are coupled by pins 19 to the side of a flange portion 12a formed in the central portion of the sleeve shaft 12. The second planetary gears 14 are not coupled to the sun gear 13 or housing 21 but mesh with a free second ring gear. A steering shaft 25 is coupled to the axial rear end portion of the sleeve shaft 12 and a steering gear (not shown) is coupled to the pinion output shaft 9 by means of a rack 17.
FIG. 7 shows an illustrative torque detector. Shown by 30 is a potentiometer type stroke sensor. A detection lever 30a on this potentiometer 30 is forced and fixed in a hole 33a at the right-hand axial end of a spool 33. A spring for holding the spool 33 on the neutral position is inserted into a receptacle 33b on the lefthand side of the spool 33 and into a hole 32 formed in the housing 21. The potentiometer 30 is secured to the housing 21 by fastening screws 36 and 37. A pin insertion hole 31 is formed in the central portion of the hole 32. An action lever pin 35 is inserted through the center of the spool 33, and the tip 35a of the pin 35 engages a cutout 16a formed in a selected portion of the circumference of the second ring gear 16. In the torque detector having the composition described above, the action lever pin 35 is driven either rightward or leftward together with the second ring gear 16, and in response to this movement, the spool 33 and detection lever 30a make either a rightward or a leftward movement.
The operation of the prior art torque detector will proceed as follows. Suppose here the case that with a load torque being transmitted to the pinion shaft 9 via the steering gear (not shown) and rack 17, the steering wheel is handled to have a steering torque act on the steering shaft 25.
(a) If the steering torque is smaller than the load torque, the steering torque on the wheel is transmitted to the input end (right-hand end ) of the sleeve shaft 12 whereas the load torque is exerted on the pinion shaft 9. As a result, the torque detecting torsion bar 11 undergoes a torsional deformation that is proportional to the difference between the two torques. In response to this torsional deformation, the second planetary gears 14 which are coupled to the flange portion 12a of the sleeve shaft 12 by pins 19 will tend to effect a rotational displacement by rotating the sun gear 13 that meshes with them. However, since the sun gear 13 engages the pinion shaft 9 via the first planetary gears 10 and the load torque acting on this pinion shaft 9 is greater than the steering torque, the pinion shaft 9 is locked so as to prohibit the rotation of the first planetary gears 10 and hence the sun gear 13. Therefore, the second planetary gears 14 will cause a rotational displacement of the second ring gear 16. As a result, the action lever pin 35 which is in engagement with the second ring gear 16 is also displaced in the direction in which said ring gear is rotated. In synchronism with this displacement of the lever pin 35, the detection lever 30a on the potentiometer 30 is driven to be displaced in the same direction. Consequently, the torsional deformation of the torsion bar 11 proportional to the steering torque is finally converted to a stroke displacement of the potentiometer. If this stroke displacement is picked up in the form of an electrical the torque detector produces linear characteristics in that the resulting electrical output is proportional to the steering torque.
(b) If the steering torque is very small, it will not produce any torsional deformation of the torsion bar 11 that will eventually cause a rotational displacement of the second ring gear 16, and this makes the detection of the steering torque impossible. Therefore, the torque detector has a dead zone in the range where the steering torque is too small to produce a torsional deformation of the torsion bar 11.
(c) If the steering torque is excessive, its detection is also impossible because the stroke of the action lever pin 35 is restricted by the inside diameter of the pin insertion hole 31 and detection of any steering torque that corresponds to that deformation of the torsion bar which exceeds this range is theoretically impossible. In other words, there is also an upper limit for the detection of steering torque.
FIG. 8 shows a modeling of the response characteristics of the prior art torque detector that summarizes the discussion made in (a) to (c).
This prior art torque detector has the following disadvantages:
(1) Linearity in its output characteristics is not attainable over a broad operating range;
(2) Being of the potentiometer type, the detector is subject to variations in contact resistance due to such factors as eccentricity of the axial center that may occur during assembly or in other situations and the resulting drift in output signals will cause variations in the output of the detector;
(3) Since the detection lever 30a on the potentiometer 30 is forced and fixed in the hole 33a to have engagement with the spool 33, any offset in the center of the hole 33a that is introduced during working will cause the detection lever 30a to be driven eccentrically; if the amount of this offset is great, malfunctioning of the detector will occur and even if the amount of offset is small, both the lever 30a and the housing 21 will wear excessively to cause troubles such as poor brush contact, leading to reduced reliability of the torque detector.